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I'm 3 quarters through Bach's Menuet 114, but I don't understand what's written in measure 25 as seen in the picture.In the bass clef: why is that quarter rest sign there and how about the 2 half notes ? Do I have to sustain the first while starting the second half note ? Mathematically it's not correct.Thanks for your help !

Mathematically it is correct. Three beats to the measure. And yes, you sustain the first note when playing the second. Specifically:

So in the left hand, the B starts on the first beat and last for two quarter notes. On the second beat you add the D, also two quarter notes. And on the third beat, you release the B and play it again for one quarter note.

The rest is there as a guide to the fact that there are two voices in that hand and one (the one that will play the D) is silent for the first beat. It's very common in polyphony.

Ok, thanks for that rapid response !I now fully understand what's there.To play it correctly will be a whole different affair ...I have never played sustained notes in one hand while starting another note.

As Andy said. Imagine that you have three singers. The notes in the treble clef are the sopranos. The stem down notes in the bass clef are the bass, and the stem up in the bass clef are the tenors. In piano we tend to think vertically, meaning in terms of harmony. But there is a horizontal layer too for this kind of music. If you are aware of three voices singing three melodies then you may shade your playing that way. If you are actually seeing three melodies, then the rests help you stay oriented.